Typically, a tidal turbine consists of a mechanical receiver or turbine, with variable speed, able to be set into rotation by hydraulic stream flow, for example a wave or a current in a stream, and of an alternator connected to the mechanical receiver, which converts the mechanical power generated by the hydraulic stream flow into electric power. An alternator typically comprises a rotor which is mechanically connected to the turbine and set into rotation by the latter and one or several stators, the rotation of the rotor creating an electromagnetic field by induction. Thus, the mechanical energy provided to the rotor is transformed into AC electric energy. The voltage and the electric frequency provided are proportional to the speed of rotation of the rotor.
The alternator provides at the output an electric AC current with variable frequency, the frequency being dependent on the speed of rotation of the turbine which depends on the flow rate of the hydraulic stream, itself variable. Now electricity distribution and transport networks operate at a set frequency, for example 50 Hz in Europe and 60 Hz in the USA.
In order to allow electricity to be provided for a transport or distribution network, the coupling of such an alternator is known with an electric energy converter, capable of converting the electric signal from the alternator into an electric signal with a set voltage and a set frequency, ready to be provided to the distribution or transport network.
Presently, connecting an energy converter with an alternator and therefore with a tidal turbine is known, in order to draw the best possible yield of the stream flow passing through each tidal turbine and to convey a maximum of electric power towards the distribution or transport network. Nevertheless, the integration of one power converter per device for producing electricity, either immersed or partly immersed, is expensive and restrictive, in particular because of the requirement of ensuring the seal and an increased reliability level of such a power converter; alternatively, the installation of an emerged power converter at each machine is not acceptable, neither by the public (visual impact), nor by the authorities (restriction to navigation and to fishing, hazard). Further, the maintenance of such an immersed or partly immersed power converter is very expensive, since it requires means for intervening at sea, in particular for lifting underwater caissons which are generally very heavy.
An alternative solution consisting of placing for each alternator, the associated power converter on the ground, may be contemplated but is particularly expensive, since this requires the use of a cable for transporting an electric AC signal at a variable frequency by for electricity producing device.
It is desirable to make the electric energy producing installations based on tidal turbines, less expensive and easier to maintain.